Beta cells (βcells) are a type of cell found in the pancreatic islets of the pancreas. They make up 65-80% of the cells in the islets. The primary function of a beta cell is to store and release insulin—a hormone that reduces blood glucose concentration. Beta cells can respond quickly to spikes in blood glucose concentrations by secreting some of their stored insulin, while simultaneously producing more.
Insulin release is mediated by ion channels. Voltage-gated calcium channels and ATP-sensitive potassium ion channels are embedded in the cell surface membrane of beta cells. These ATP-sensitive potassium ion channels are normally open and the calcium ion channels are normally closed. Potassium ions diffuse out of the cell, down their concentration gradient, making the inside of the cell more negative with respect to the outside (as potassium ions carry a positive charge). At rest, this creates a potential difference across the cell surface membrane of −70 mV.
When the glucose concentration outside the cell is high, glucose molecules move into the cell by facilitated diffusion, down their concentration gradient through the GLUT2 transporter. Since beta cells use glucokinase to catalyze the first step of glycolysis, metabolism only occurs around physiological blood glucose levels and above. Metabolism of the glucose produces ATP, which increases the ATP to ADP ratio.
The ATP-sensitive potassium ion channels close when this ratio rises. This means that potassium ions can no longer diffuse out of the cell. As a result, the potential difference across the membrane becomes more positive (as potassium ions accumulate inside the cell). This change in potential difference opens the voltage-gated calcium channels, which allows calcium ions from outside the cell to diffuse into the cell, down their concentration gradient. When the calcium ions enter the cell, they cause vesicles containing insulin to move to, and fuse with, the cell surface membrane, releasing insulin by exocytosis.
Diabetes mellitus is a disease caused by the loss or dysfunction of insulin-producing beta cells in the pancreas. Specifically, in type 2 diabetes mellitus, beta cells exhibit an impaired capacity to compensate for increased insulin demand, a defect that has been ascribed to both inadequate cellular capacity to secrete insulin and beta cell death. In addition, diabetes can be accompanied by peripheral insulin resistance.
This impairment in glucose-stimulated insulin secretion has been attributed to defects in glucose sensing, mitochondrial dysfunction and oxidative stress. Results of other studies suggest that defects in multiple cellular processes can compromise beta cells function and can be a factor to induce diabetes mellitus. A simultaneous loss of beta cell function and identity could be explained by reduced expression of a central transcriptional regulatory network involved in beta-cells differentiation and maintenance. Recent studies suggest that dysregulation of the beta cells' differentiation state is among the earliest events marking the progressive failure of beta cells in diabetes (FIG. 1).
Therefore, innovative strategies for diabetes therapy aim to replace lost or damaged insulin-producing beta cells by reprogramming of other cell types towards beta cells lineage. Previous studies concentrated on the reprogramming of embryonic pluripotent or induced pluripotent stem cells towards beta cells using various factors (FIG. 2) (Kroon, 2008; Greggio, 2013). These differentiated cells, however, often lack much of the structure and markers that beta cells need to perform their necessary functions. Examples of the anomalies that arise from beta-like cells differentiated from progenitor cells include a failure to react to environments with high glucose concentrations, an inability to produce necessary beta cell markers, and abnormal expression of glucagon along with insulin.
Efficient and reproducible differentiation of initially unmodified autologous adult stem cells into glucose-responsive, insulin-producing beta cells has not been done. Most of the previous studies for production of beta cells have been based on the attempts to up-regulate one or two insulin inducing factors and had only marginal success.
Therefore, what is still needed in the art are robust methods for generation of pancreatic beta-cells by genetic re-programming of primary unmodified adult adipose derived stem cells (ADSCs) or other adult stem cells.